1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display device. For example, the present invention relates to a method for manufacturing an electro-optical device typified by a liquid crystal display panel having a circuit using a thin film transistor (hereinafter, a TFT) and a method for manufacturing an electronic device provided with such an electro-optical device as a component.
2. Description of the Related Art
In recent years, attention has focused on a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface. Thin film transistors are widely applied to electronic devices such as ICs and electro-optical devices, and in particular, their rapid development as switching elements for image display devices is desired.
A liquid crystal display device is known as an example of the image display devices. Compared to passive matrix liquid crystal display devices, high-definition images can be obtained with active matrix liquid crystal display devices; therefore, the active matrix liquid crystal display devices have become widely used. In the active matrix liquid crystal display devices, when pixel electrodes arranged in matrix are driven, a display pattern is formed on a screen. In more detail, when voltage is applied between a selected pixel electrode and a counter electrode that corresponds to the selected pixel electrode, a liquid crystal layer provided between the pixel electrode and the counter electrode is optically modulated, and this optical modulation is recognized as a display pattern by a viewer.
The application range of such active matrix electro-optical devices is expanding, and demands for high-definition, a higher aperture ratio, and high reliability are increasing as a screen size gets larger. At the same time, demands for improvement in productivity and cost reduction are increasing.
The cost for materials is increased as the size of the panel gets larger. In particular, a liquid crystal material provided between a pixel electrode and a counter electrode is expensive.
In the case of using a liquid crystal injection method, sealing of liquid crystal requires a complex process such as drawing of a sealant, attachment of a counter substrate, division of substrates, injection of liquid crystal, and sealing of an inlet for injecting liquid crystal. In particular, as a panel size gets larger, it becomes difficult to fill a region surrounded by the sealant (including at least a pixel portion) with liquid crystal since liquid crystal is injected using a capillary phenomenon. When liquid crystal is injected using a capillary phenomenon, a larger amount of liquid crystal than that to be injected from the liquid crystal inlet is used in vain.
Further, when a liquid crystal injection method is used, two substrates are attached to each other and divided, and then, a liquid crystal material is injected from a liquid crystal inlet formed on the divided surface. At this time, a path of the liquid crystal material extending from the liquid crystal inlet to a pixel portion is also filled with the liquid crystal. Further, when a driver circuit portion and a pixel portion are provided over one substrate, not only the pixel portion but also a region overlapping with the driver circuit portion is filled with the liquid crystal in some cases. In such a manner, a region except the region to be a display portion is also filled with the liquid crystal material.
In addition, an extremely large amount of liquid crystal flows in the path of the liquid crystal material extending from the liquid crystal inlet to the pixel portion especially around the liquid crystal inlet, compared to other portions in the panel. Therefore, there is a concern that around the inlet, the surface of an alignment film is changed due to friction caused by injecting the liquid crystal, and orientation of liquid crystal molecules is disordered as a result.
Further, in a liquid crystal injection method, a step of sealing the liquid crystal inlet is necessary after the liquid crystal injection.
The present applicant propose a technique of attaching a pair of substrates to each other under reduced pressure after dripping liquid crystal in Reference 1 (U.S. Pat. No. 4,691,995).